The present invention relates to integrated circuit (IC) packaging and, more specifically, to a semiconductor device with a heat-dissipating lead frame.
A lead frame is a collection of metal leads and possibly other elements (e.g., die paddle or flag and power bars) that is used in semiconductor packaging for assembling one or more IC dies into a single packaged semiconductor device. Prior to assembly into a packaged device, a lead frame may have support structures (e.g., a rectangular metal frame) that keep those elements in place. During the assembly process, the support structures may be wholly or partially removed. As used herein, the term “lead frame” may be used to refer to the collection of elements before assembly or after assembly, regardless of the presence or absence of those support structures.
FIG. 1A is a plan view of a conventional lead frame 100. FIG. 1B is a cross-sectional view of the lead frame 100 of FIG. 1A along cut line Y-Y. The lead frame 100 is a patterned sheet metal cut-out that includes a die pad or flag 101 and lead fingers 102(1)-102(44) that surround the die pad 101. The lead frame 100 may be used in IC packages such as quad flat packages (QFP). The die pad 101 is used for the mounting of at lease one IC die (not shown) during assembly, where the die is attached to the die pad 101 by, for example, an adhesive (not shown). The lead fingers 102, which will be transformed into corresponding leads during assembly, are for providing electrical connections between device-internal components of the die and device-external components.
The lead frame 100 has eleven lead fingers 102 per side. The lead fingers 102(11*(i−1)+1)-102(11*i) are supported by a corresponding dam bar 103(i), where i is an integer from one to four, inclusive. For example, the lead fingers 102(1)-102(11) are supported by the dam bar 103(1), and the lead fingers 102(12)-102(22) are supported by the dam bar 103(2). More generally, the relationship for a QFP can be described as dam bar number i supporting lead finger numbers N*(i−1)+1 to N*i, where N is the number of lead fingers per side. The dam bars 103 form a rectangle that is supported at its corners by corresponding tie bars 104(1)-104(4), which hold the dam bars 103 and lead fingers 102 in place relative to the die pad 101. Each tie bar 104 has a corresponding tie-bar down-set 105, which comprises two bends that allow the die pad 101 to be on a first plane and the lead fingers 102 to be on a second plane that is parallel to and at a different elevation than the first plane, as can be seen in FIG. 1B.
During assembly, a die (not shown) is mounted on the die pad 101 and then electrically connected to the lead fingers 102 with, for example, bond wires as is known in the art.
Following wire bonding, the assembly comprising the die, the lead frame 100, and the bond wires is mostly encapsulated in an encapsulant such as a molding compound, leaving the distal ends of the lead fingers 102 exposed. In some packages, the bottom of the die pad 101 is also left exposed, which allows for more-efficient dissipation of heat from the die.
In a typical assembly process, the lead frame 100 is just one lead frame in a one- or two-dimensional array of lead frames, where two adjacent lead frames in the array may share additional support structures (not shown). After encapsulation, the assembly is singulated into individual IC devices by sawing or laser cutting, which also removes the additional support structures, if any, and cuts the inter-lead segments of dam bars 103 to electrically isolate the lead fingers 102 from each other and from the tie bars 104. The lead fingers 102 of each IC device may also be trimmed and formed into shapes, such as so-called gull wings or j-leads, to form the leads of the packaged semiconductor device that is then ready for mounting on a printed circuit board (PCB). The bottom of the die pad 101 may be left exposed, or may be mounted on a heat sink or onto heat-conducting elements on the PCB.
As integrated circuits are becoming more complex and including more transistors, they are generating more heat. Accordingly, it would be advantageous to have a lead frame for a semiconductor device package that is good at dissipating such heat.